Fuel cell system

ABSTRACT

A fuel cell system is provided which is capable of changing a fuel cartridge while the power generation is active in a fuel cell. For this purpose, the fuel cell system having a fuel cell is provided in a detachable manner, and the system includes: a fuel cartridge which stores fuel supplied to the fuel cell; a fuel sub-tank which stores the fuel delivered from the fuel cartridge; and a buffer tank which stores the fuel which is delivered from the fuel sub-tank and diluted to a predetermined concentration. There is provided a tank communicating passage through which gas freely enters and exits between the top of fuel sub-tank and the buffer tank.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell system and, moreparticularly, to a fuel cell system that generates electric power bysupplying the liquid fuel thereto.

2. Description of the Related Art

A fuel cell is a device that generates electricity from hydrogen andoxygen so as to obtain highly efficient power generation. A principalfeature of a fuel cell is its capacity for direct power generation whichdoes not undergo a stage of thermal energy or kinetic energy as inconventional power generation. This presents such advantages as highpower generation efficiency despite the small scale setup, reducedemission of nitrogen compounds and the like, and environmentalfriendliness on account of minimal noise or vibration. A fuel cell iscapable of efficiently utilizing chemical energy in its fuel and as suchenvironmentally friendly. Fuel cells are therefore envisaged as anenergy supply system for the twenty-first century and have gainedattention as a promising power generation system that can be used in avariety of applications including space applications, automobiles,mobile devices, and large and small scale power generation. Serioustechnical efforts are being made to develop practical fuel cells.

Of various types of fuel cells, a polymer electrolyte fuel cell excelsin its low operating temperature and high output density. Recently,direct methanol fuel cells (DMFC) are especially attracting theattention as a type of polymer electrolyte fuel cell. In a DMFC,methanol water solution as a fuel is not reformed and is directlysupplied to the anode so that electricity is produced by anelectrochemical reaction induced between the methanol water solution andoxygen. Discharged as reaction products resulting from theelectrochemical reaction are carbon dioxide emitted from the anode andgenerated water emitted from the cathode. Methanol water solution has ahigher energy density per unit volume than hydrogen. Moreover, it issuitable for storage and poses little danger of explosion. Accordingly,it is expected that methanol water solution will be used in powersupplies for automobiles, mobile devices (cell phones, notebook personalcomputers, PDAs, MP3 players, digital cameras, electronic dictionariesand books) and the like.

RELATED ART LIST

-   (1) Japanese Patent Application Laid-Open No. 2005-108811.

In the fuel cell system, as in Reference (1), containing DMFC, the airserving as an oxidant is supplied to the cathode and therefore theinside of fuel supply passage, including the interior of a buffer tank,at a fuel cell apparatus side is at high pressures. Thus, when the pumpstops, there are cases where diluted methanol water solution flowsbackward into the fuel supply passage. When the methanol water solutionflows back into the fuel supply passage, the diluted methanol watersolution returns to the buffer tank even if the pump operates next time.This causes a problem where the concentration drops rapidly and thepower generation capacity drops. The problem like this also occurs whenthe air bubble is mixed into the fuel supply passage, and a problem mayarise where the air bubble enters the fuel supply passage at the time ofreplacement of a fuel cartridge and the like.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoingcircumstances and a general purpose thereof is to provide a fuel cellsystem capable of supplying liquid fuel to a fuel cell. Anotheradvantage of the present invention is that it provides a fuel cellsystem in which the leakage of fuel from a buffer tank is prevented.

One embodiment of the present invention relates to a fuel cell system.This fuel cell system is a fuel cell system that supplies liquid fuel toa fuel cell and operates the fuel cell, and the system includes: a firstfuel storage, provided detachably, which stores the liquid fuel suppliedto the fuel cell; a first fuel supply means which delivers the fuel cellstored in the first fuel storage; a second fuel storage which stores theliquid fuel delivered by the first fuel supply means; a second fuelsupply means which delivers the liquid fuel stored in the second fuelstorage; and a fuel detection means, provided in the second fuelstorage, which detects an amount of the liquid fuel stored in the secondfuel storage, wherein a free path through which gas inside the secondfuel storage is sucked in and discharged is provided on top of thesecond fuel storage.

According to this embodiment, even when the first fuel storage(so-called a fuel cartridge) is removed from the fuel cell system, somefuel is stored in the second fuel storage and therefore the state ofpower generation can be remained active. Although the air bubble causedwhen the first fuel storage is mounted or the like may enter the secondfuel storage, the air bubble is eliminated in the second fuel storage.Thus such bubble can be prevented from entering the fuel supply means.Since the gas in the second fuel storage flows through the free path,the back flow or the like of the fuel is hardly likely to occur. Hence,the concentration of the fuel supplied to a fuel cell can be keptconstant and the fuel cell can be operated stably.

In the case where the second fuel storage is a container of typical orstandard type, a water-level sensor (liquid-level sensor) for detectingthe water level inside may serve as the fuel detection means, forexample. Also, some mechanism that does not detect the water levelconstant but one, such as a limiter, capable of detecting whether it isbelow a predetermined threshold or not may serve as the fuel detectionmeans. Thereby, a case like the imminent situation where the fuel cellsystem will cease to operate unless the fuel cartridge is replaced anewimmediately can be notified to a user.

In the above embodiment, the free path may be tube-shaped such that oneend thereof is open, and the free path may be such that inner volumethereof is greater than or equal to-one-time liquid cell capacity by thefirst fuel supply means. According to this, the liquid fuel thatoverflows from the second fuel storage can be retained inside the freepath, so that the leakage of liquid to the outside of a system issuppressed.

In the above embodiment, a vapor-liquid separating structure may beprovided in an end of the free path. With the provision of thisair-liquid separating structure, the fuel is not leaked out of the freepath and the fuel gas of less than or equal to a predeterminedconcentration is released to the outside, so that the safety of the fuelcell system can be raised. More specifically, the vapor-liquidseparating structure may have either an absorber which collects theliquid fuel or a filter which absorbs the fuel in exhaust gas. In thelatter case, the filter may be activated carbon.

In the above embodiment, a fuel cell system may further include a thirdfuel storage, connected with the second fuel supply means, which mixesemission material from the fuel cell with the liquid fuel delivered bythe second fuel supply means, which prepares and stored a fuel to besupplied to the fuel cell, and which discharges unwanted gas componentto the outside of the fuel cell system, wherein the end of the free pathmay be connected with a gaseous layer part of the third fuel storage.

By employing this structure, when the fuel cell is a fuel cell of thetype such as DMFC, in which the liquid fuel is directly supplied to thefuel cell, the liquid fuel, whose concentration of fuel has beenreduced, and the carbon dioxide are discharged from the anode of thefuel cell whereas the air, whose concentration of oxygen has beenreduced, and the water are discharged from the cathode thereof. In thethird fuel storage, the liquid components of this emission material (theliquid fuel whose concentration of fuel has been reduced and the water)and the fuel from the second fuel storage are mixed together, and theliquid gas components of the emission material (the carbon dioxide andthe air whose concentration of oxygen has been reduced) are released tothe outside. Thus the fuel can be utilized efficiently.

It is to be noted that any arbitrary combination or rearrangement of theabove-described structural components and so forth are all effective asand encompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be sub-combinationof these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of examples only, withreference to the accompanying drawings which are meant to be exemplary,not limiting and wherein like elements are numbered alike in severalFigures in which:

FIG. 1 is a perspective view of a fuel cell system according to a firstembodiment of the present invention;

FIG. 2 schematically illustrates a structure of a fuel cell systemaccording to a first embodiment of the present invention;

FIG. 3 illustrates in detail a structure of a sub-tank in a fuel cellsystem according to a first embodiment of the present invention;

FIG. 4 schematically illustrates a structure of a fuel cell systemaccording to a second embodiment of the present invention;

FIG. 5 schematically illustrates a gas circulation tube;

FIG. 6 schematically illustrates a structure of a fuel cell systemaccording to a third embodiment of the present invention;

FIG. 7 illustrates a configuration, having a vapor-liquid separationstructure, in a fuel cell system according to a third embodiment of thepresent invention; and

FIG. 8 illustrates another structure of a vapor-liquid separator used ina fuel cell system according to a third embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention.

A structure of a fuel cell system 100 according to the present inventionwill now be described in detail with reference to drawings.

First Embodiment

FIG. 1 is a perspective view of a fuel cell system 100 according to afirst embodiment of the present invention. FIG. 2 illustrates astructure of the fuel cell system 100. A fuel cell system 100 accordingto the present embodiment uses methanol as a liquid fuel and generateselectric power in a fuel cell by an electrochemical reaction inducedbetween this methanol and air serving as an oxidant. This system is aso-called a direct methanol fuel cell (DMFC), and the overall dimensionthereof is configured compactly so that it can be suitably used as apower supply for a portable notebook personal computer or the like.

The fuel cell system 100 is structured such that inside a casing 10 asshown in FIG. 1 a stack of fuel cells 20 is mounted on one side thereofin a longitudinal direction, a fuel cartridge 30 connected detachablywith the fuel cell system 100 is mounted on-the other side, and anauxiliary unit 40 is mounted approximately in the center. A control unitand a secondary cell, both not shown in FIG. 1, are provided within acradle 50 on which a notebook personal computer is placed.

Adjacent to the fuel cartridge 30 there are provided a fuel sub-tank 80,which is a second fuel storage, and a buffer tank 90, which is a thirdfuel storage. Pure methanol or high concentration methanol watersolution stored in a fuel bag (first fuel storage) within the fuelcartridge 30 is introduced into the buffer tank 90 via the fuel sub-tank80 and then diluted, by the buffer tank 90, into a predeterminedconcentration of 1 mol/L. That is, the fuel sub-tank 80 has functions ofnot only detecting that the fuel level has reached the zero but alsoeliminating the air (gas components) mixed in with the fuel supplypassages 34 and 82 at the time when the fuel cartridge 30 is mounted orremoved. Also, the buffer tank 90 has not only a function of adjustingthe concentration of fuel but also a function of a vapor-liquidseparator in which the gas components discharged from the fuel cell 20is discharged externally from said fuel cell system 100 (the detail willbe discussed later).

The auxiliary unit 40 includes a methanol pump (first liquid pump) 41which supplies fuel from the fuel bag 32 to the fuel sub-tank 80, amethanol pump (second liquid pump) 42 which supplies fuel from thebuffer tank 90 to the fuel cell 20, a methanol pump (third liquid pump)43 which supplies fuel from the buffer tank 90 to the fuel cell 20, andan air pump 44 which supplies oxygen, which is air in the presentembodiment. This auxiliary unit 40 is mounted between the fuel cell 20and the fuel storage comprised of the fuel cartridge 30, the fuelsub-tank 80 and the buffer tank 90. This structure is implemented forthe purpose of having minimum length of the fuel supply passages 34, 82,84, 91 and 92 to save the space and increase the space utilizationefficiency and supplying promptly the high concentration methanolsupplied intermittently to the fuel cell 20.

The auxiliary unit 40 includes: a vapor-liquid separator 45 which mixesan anode discharge (waste methanol+carbon dioxide) composed principallyof the liquid discharged from an anode 21 side of a fuel cell 20 with acathode discharge (exhaust air+generated water) composed principally ofgas discharged from a cathode 22 side thereof and separates the mixtureinto a gas component and a fluid component; and a cooler 46 whichdistributes the gas component and the fluid component separated by thevapor-liquid separator through different pipings and cools thedischarges of the fuel cell 20 by a cooling fan 47 that discharges theair inside the fuel cell system 100. And the auxiliary unit 40 includingthe vapor-liquid separator 45 and the cooler 46 is mounted between thefuel cell 20 and the buffer tank 90. In this manner, the vapor-liquidseparator 45 having a function of separating vapor and liquid isinstalled before the buffer tank 90 as well as the cooler 46 (namely,located upstream of the buffer tank 90). Thus, the anode discharge andthe cathode discharge in which liquid and gas are mixed are united andthen the liquid component and the gas component are distributedrespectively to a liquid component passage 46 a and a gas componentpassage 46 b so as to be cooled, so that the heat exchange efficiencycan be improved as compared to when the fluid with the gas and theliquid mixed together is cooled.

The gas component out of the discharges of the fuel cell 20 recovered bythe buffer tank 90 is released, outside the fuel cell system 100,through a gas component exhaust passage 93. In such a case, the gascomponent exhaust passage 93 is provided as long as possible so that thegas component is not released outside, and it is preferred that anexhaust filter 94 be provided at an exist. In view of possibility thatthe amount of generated water produced by the fuel cell 20 is greaterthan the amount of water vapor discharged from the buffer tank 90 andtherefore the fuel (methanol water solution) circulating within the fuelcell system 100 overflows, the buffer tank 90 and the fuel sub-tank 80are connected with each other through a piping (tank communicatingpassage 95) provided above them. When the buffer tank 90 overflows, thefuel sub-tank 80 plays a role of the buffering in the buffer tank 90 andat the same time the fuel is supplied from the fuel cartridge to thefuel sub-tank 80. And if the pressure in the fuel sub-tank 80 risestemporarily, the buffer tank 90 plays a role of allowing the pressure ofthe fuel sub-tank 80 to escape. A check valve 96 is provided between thefuel sub-tank 80 and the buffer tank 90, and it is so structured thatthe diluted methanol water solution does not flow backward from the fuelsupply passage 91 to the fuel supply passage 84, namely from the buffertank 90 to the fuel sub-tank 80, unless it overflows via the tankcommunication passage 95. Cartridge joints 36 and 86 are providedbetween the fuel bag 32 and the fuel sub-tank 80, and the fuel supplypassage 34 and the fuel supply passage 82 are connected to each otherthrough these cartridge joints 36 and 86. In order that a safetymechanism for recovering the leakage of fuel, at the time of insertingor removing a cartridge, a locking mechanism for the joint or the likemechanism can be provided in the housing main body side, this joint partis such that the cartridge joint 36 in the fuel cartridge 30 side is amale whereas the cartridge joint 86 in the fuel sub-tank 80 side is afemale. In this manner, the female structure allows the assembly of morecomplicated mechanisms and realizes a simple structure in the fuelcartridge 30 side, thus achieving advantageous aspects in terms of thesize and cost.

To detect the status of whether-the fuel cartridge is being inserted orremoved, a limiter LT is provided in the housing main body of the fuelcell system 100 in contact with the fuel cartridge 30. This structureallows detecting whether the fuel cartridge 30 is normally fit into thefuel cell system 100 or not, so as to make sure that the fuel is notleaked from the cartridge joints 36 and 86 while in use. The means fordetecting whether it is inserted or removed is not limited to thelimiter LT, and a structure may be such that an IC chip or the like isembedded in a predetermined position of the fuel cartridge 30 so as todetect the position of the IC chip and at the same time the information,on the fuel cartridge, such as volume, concentration, fuel type andserial number is communicated between the fuel cell system 100 and thecontrol unit.

The fuel supply passage 34 in the fuel cartridge 30 has its inletpositioned in the bottom of the fuel bag 32, and is so arranged as tomove upwards along the side of wall within the fuel cartridge 30 and isthen connected with the cartridge joint 36. A fuel confirmation windowis set up in an upper part, namely part of upper hem, of the fuelcartridge so that the fuel supply passage 34 is visible. It is desirablethat transparent material such as Teflon (registered trademark) tube beused to form the fuel supply passage to confirm the interior of the fuelsupply passage 34 from this fuel confirmation window 38. The fuel bag 32is a container such that the volume thereof can be varied and a smallamount of gas such as air is presealed inside together with the fuel.Hence, when the remaining fuel stored in the fuel bag 32 gets small, theboundary between liquid phase and gaseous phase can be visibly verified.The confirmation will be further facilitated if the fuel is coloredbeforehand.

The above flow of the fuel is summarized as follows. The highconcentration methanol or pure methanol in the fuel bag 32 isdistributed through the fuel supply passage 34 so as to be supplied tothe housing main body of the fuel cell system 100. The fuel cartridge 30and the housing main body are connected with each other by way of thecartridge joints 36 and 86, and the high concentration methanol in thefuel bag 32 is supplied to the fuel sub-tank 80 by the suction force ofthe methanol pump 41 provided in the fuel supply passage 82 which isconnected from the cartridge joint 86 to the fuel sub-tank 80. If thegas is mixed in with the fuel supply passages 34 and 82 from thecartridge joints 36 and 86 at the time then the fuel cartridge 30 ismounted or removed, such gas is eliminated by this fuel sub-tank 80.Hence, a structure is such that such gas as air bubbles is not mixedinto a buffer tank 90 side from the fuel sub-tank 80.

As shown in FIG. 3, a liquid-level sensor 81 that detects fuel runout isprovided in the fuel sub-tank 80 in a position where it is located ½ ormore of the height a of the tank. A structure is such that when it isdetected that the water level of fuel in the fuel sub-tank 80 hasreached the position of the liquid-level sensor 81 or below, themethanol pump 41 is driven and high-concentration fuel is added to thefuel sub-tank 80 from the fuel bag 32. If the water level of fuel in thefuel sub-tank 80 is not recovered after the methanol pump 41 has beendriven for a predetermined period of time, an alarm will be displayed toa user indicating that the fuel has run out. If the fuel water level ofthe fuel cartridge 30 is not recovered even after the time forreplacement set for the change of the fuel cartridge 30 has passed andthe alarm to a user indicating that the fuel has run out was issued, thesystem will cease to operate. It is necessary that this liquid-levelsensor 81 is provided in a position such that a sufficient amount offuel to operate a system is still stored while the fuel cartridge 30 isreplaced anew. In the present embodiment, the time required for thecartridge replacement is set to approximately 5 minutes, and the amountof fuel needed to operate the system while the cartridge is replacedanew is about 5 cc. Also, in the present embodiment the liquid-levelsensor 81 is placed in a position where it is located at ½ or more ofthe height of the tank. The high concentration methanol in the fuelsub-tank 80 is supplied to the buffer tank 90 by the suction force ofthe methanol pump 42 provided in the fuel supply passage 84. The fuelsupply passage 84 is connected with the fuel supply passage 91 by way ofthe check valve 96, and a structure is such that, as a steady state, thediluted methanol water solution in the buffer tank 90 side does not flowback into the fuel sub-tank 80 from the check valve 96.

In the sub-tank 80, a gas intake/exhaust opening 101 communicated withthe tank communicating passage 95 is provided on the top surface of acontainer, thus realizing a structure such that the gas inside thecontainer can freely enter and exit. By employing this structure, thepressure inside the container does not get pressurized nor becomesnegative-pressure even when the liquid level varies. As a result, thesafety of the fuel sub-tank 80 is raised. Also, abnormal operationswhere, for example, the liquid fuel flows back into the fuel supplypassage 82 and the liquid fuel flows into the fuel supply passage 84 atthe timing other than a predetermined timing can be prevented.

A description will now be given of variation in the liquid level of fuelin the fuel sub-tank 80. According to the state of power generation inthe fuel cell 200, a predetermined amount v of liquid fuel is suppliedintermittently to the fuel cell 20 from a fuel exhaust port 104 locatedat an end of the fuel supply passage 84. In what is to follow, assumethat the fuel added amount at one time is v and the amount of liquidlevel of fuel that descends in the container at one time of fueldischarge is y. When it is detected that the water level of fuel in thefuel sub-tank 80 has become the height of the liquid-level sensor 81 orbelow as a result of the fuel discharge, the methanol pump stops andthen the methanol pump 41 starts. As a result, the liquid fuel is addedfrom a fuel filler port 102 located at an end of the fuel supply passage82 so that the water level of fuel in the fuel sub-tank 80 is in aposition of (½)·y above the height of the liquid-level sensor 81. Inthis manner, the methanol pump 41 and the methanol pump 42intermittently operate at asynchronous timing, and the water inside thefuel sub-tank 80 varies within a range of y wherein y indicates a rangeof variation in the liquid level. The methanol pump 41 and the methanolpump 42 are controlled in an asynchronous manner. Thus, the fuel cell 20can be operated without disabling the supply of liquid cell even in thecase when the quantity accuracy of the methanol pump 41 is low due tothe disturbance such as variation in pressure of the methanol pump 41required of fuel discharge from the fuel cartridge 30 as a result ofvariation in the remaining amount of the liquid fuel and the timedegradation.

It is also preferred that the gas intake/exhaust opening 101 be providedin a position higher by at least y from the maximum point of the liquidlevel variation range y. That is, with reference to FIG. 3, it ispreferable that the length H₁, which is the distance from the maximumpoint of the liquid level variation range y to the gasintake/exhaust-opening 101), is greater than y. With this configuration,a gas layer can be kept in an upper part of fuel in the fuel sub-tank 80even when the fuel is added once by the methanol pump 41. Moreover, whenthe diameter of the fuel sub-tank 80 is denoted by x, it is preferablethat the gas intake/exhaust opening 101 is provided in a position higherthan the maximum point of the liquid level variation range y by at leastx·tan θ. That is, with reference to FIG. 3, the length H₁ is greaterthan x·tan θ. The angle θ is preferably 45 degrees. By employing thisstructure, in a case when the fuel sub-tank 80 is tilted by the angle θand at the same time the gas intake/exhaust opening 101 is soaked in theliquid fuel, the resulting lowered level of the liquid fuel is notdetected by the liquid-level sensor 81. Thus, the operation stability inthe fuel cell system 100 can be improved.

It is also preferred that the fuel filler port 102 be provided in aposition higher than the maximum point of the range y of variation inthe normal liquid level of the liquid fuel in the container. Byimplementing this structure, the capacity of fuel which can be stored inthe container can be secured and at the same time the adverse effect onthe liquid level variation range y can be reduced.

It is also preferred that the fuel exhaust port 104 be provided in aposition as closer to the bottom of the container as possible. Accordingto this structure, the fuel in the container can be discharged in anoptimal and useful manner. It is also preferred that the fuel exhaustport 104 be provided in a position lower than the minimum point of theliquid level variation range y by at least y. That is, with reference toFIG. 3, it is preferable that the length H₂, which is the distance fromthe minimum point of the liquid level variation range y to the fuelexhaust port 104), is greater than y.

By employing this structure, the fuel cell 20 can be operated even in aperiod during which the fuel cartridge 30 is being replaced anew. It isalso preferable that the fuel exhaust port 104 be provided in a positionlower than the minimum point of the liquid variation range y by at least3×y. According to this structure, it is possible to operate the fuelcell 20 during the time required for the replacement of the fuelcartridge 30 three times.

Example of Positions Where the Liquid-Sensor is Set

If the gas intake/exhaust opening 101 is in a position higher than theliquid variation range y by y and the fuel exhaust port 104 is inposition lower than the liquid variation range y by y, the height of thecontainer of the fuel sub-tank 80 will be about y+y+3y=5y. The height ofthe liquid-level sensor 81 will be about (½)·y+3y=3.5y. That is, theheight of the liquid-level sensor 81 corresponds to the position of(3.5/5)=0.7 if the height of the container is used as a benchmark.

Similar to the fuel sub-tank 80, a liquid-level sensor LS2 that detectswater shortage (fuel starvation) is also provided in the buffer tank 90in a position where it is located ⅓ or more of the height of the tank.When it is detected that the water level of fuel in the buffer tank 90has reached the position of the liquid-level sensor LS2 or below, asignal is transmitted to the control unit 60 which in turn controls eachdevice in the fuel cell system 100 in a manner such that the amount ofgenerated water produced from the fuel cell 20 increases, namely, highcurrents are outputted from the fuel cell 20. Conversely, in a casewhere the overflow occurs in the buffer tank 90, the fuel is introducedinto the fuel sub-tank 90 through the tank communicating passage 95.This tank communicating passage 95 is structured such that, as a steadystate, no difference in pressure of gaseous phase part occurs betweenthe fuel sub-tank 80 and the buffer tank 90 in the event that thepressure of gaseous phase part in the fuel sub-tank 80 rises as a resultof the insertion/removal of the fuel cartridge 30 and so forth.

The diluted methanol water solution in the buffer tank 90 is supplied tothe fuel cell 20 by the suction force of the methanol pump 43 providedin the fuel supply passage 92. In the fuel supply passage 92, a fuelfilter 97 is provided anterior to the methanol pump 43. This fuel filter97 removes or absorbs the impurities such as contaminants or cationsmixed in with the methanol solution water so as to be supplied to theanode of the fuel cell 20. Although it is acceptable that the fuelfilter 97 is installed posterior to the methanol pump 43, thecontaminants or the like in the methanol pump can be avoided if the fuelfilter 97 is provided anterior thereto. Different from the othermethanol pumps 41 and 42, this methanol pump 43 runs almost all thewhile the fuel cell system 100 is in operation. Hence, it is desirablethat the methanol filter 97 be installed posterior thereto inconsideration of the contaminants or the like.

On the other hand, the air is supplied to the cathode of the fuel cell20 by an air pump 44. The air pump 44 suctions the air inside the fuelcell system 100. However, since a filter by which to remove particlecomponents such as contaminants is provided in an opening (not shown)that introduces the air outside the fuel cell system 100 into theinterior thereof, the organic matters in the air are removed bycatalytic combustion in a posterior side of the air pump 44 in anoxidant supply passage 23 or an air filter 24 that absorbs the cationsis provided.

The waste methanol and carbon dioxide discharged from the anode 21 sideof a fuel cell 20 are discharged from an anode exhaust passage 25 to thevapor-liquid separator 45, and at the same time the exhaust air andgenerated water discharged from the cathode 22 side are discharged froma cathode exhaust passage 26. They join together at the vapor-liquidseparator 45. The mixture is separated into a liquid component and a gascomponent. Then, while the liquid component is distributed through theliquid component passage 46 a and the gas component is distributedthrough the gas component passage 46 b, the liquid component and the gascomponent are cooled by the air in the fuel cell system 100 forciblydischarged by the cooling fan 47 and are each introduced into the buffertank 90. That is, since the buffer tank 90 recovers the waste methanoland the generated water where the methanol is consumed by the electricpower cell reaction, the concentration of methanol in the buffer tank 90drops. When the methanol concentration in the buffer tank 90 drops, thevariation is caused in the voltage of a plurality of cells thatconstitute the fuel cell 20. Thus, according to the present embodiment,this variation is detected which in turn serves as a concentrationsensor instead. And when it is detected that the voltage of a pluralityof cells has become equal to or greater than a predetermined value ofvariation, a signal is sent to the control unit 60. As a result, thecontrol unit 60 drives the methanol pump 42 and replenishes the buffertank 90 with the fuel from the fuel tank 80 so as to adjust the methanolconcentration of the buffer tank 90. In cooperation with the methanolpump 42, the methanol pump 41 may operate in such a manner that the sameamount of high concentration methanol is refilled to the fuel sub-tank80 from the fuel bag 32 every time the methanol pump 42 is driven. Also,to reduce the energy consumption by the auxiliaries, a structure may,for example, be such that when the methanol pump 42 is driven threetimes, the three-fold amount of the high concentration methanol isrefilled to the fuel sub-tank 80 from the fuel bag 32.

Second Embodiment

FIG. 4 schematically illustrates a structure of a fuel cell systemaccording to a second embodiment of the present invention. The basicstructure of a fuel cell system 100 according to the second embodimentis the same as that according to the first embodiment. Hereinbelow, adescription will be given of a structure different from the firstembodiment.

In place of the tank communicating passage 95 in the first embodiment,as a free path there is provided a tube-shaped gas circulation tube 200whose one end is open.

FIG. 5 schematically illustrates a gas circulation tube 200. The gascirculation tube 200 is a small-diameter tube (for instance, the innerdiameter φ being about 1.5 mm and the length thereof being about 300 mm)having the length sufficient to store the liquid fuel. For example, itis preferable that the volume inside the gas circulation tube 200 begreater than or equal to a volume v. According to this, the liquid fuelthat overflows from the fuel sub-tank 80 can be retained inside the gascirculation tube 200, so that the leakage of liquid to the outside of asystem is suppressed.

With the material and thickness of the gas circulation tube 200 chosenappropriately, a structure is achieved such that the liquid materialevaporated inside the gas circulation tube 200 evaporates gradually fromthe surface thereof. Examples of material for the tube structured assuch include a silicon tube whose thickness is less than or equal to 0.5mm and a low-density polyethylene tube.

Third Embodiment

FIG. 6 schematically illustrates a structure of a fuel cell systemaccording to a third embodiment of the present invention. The basicstructure of a fuel cell system 100 according to the third embodiment isthe same as that according to the second embodiment. Hereinbelow, adescription will be given of a structure different from the secondembodiment.

In the fuel cell system 100 according to the third embodiment, avapor-liquid separation structure 210 is provided at the tip of a gascirculation tube 200. With the provision of the vapor-liquid separationstructure 210, the fuel gas of a predetermined concentration or less isreleased to the outside without the leakage of the liquid fuel from thegas circulation tube 200. As a result, the safety of the fuel cellsystem 100 can be raised.

FIG. 7 illustrates a configuration of the vapor-liquid separationstructure 210. In the present embodiment, the tip of the gas circulationtube 200 is inserted into a tubular fuel collector 220. The fuelcollector 220 includes a container 222, absorbent 224 and activatedcarbon 226. More specifically, the sponge-like absorbent 224 is providedaround the gas circulation tube 200 inserted into the container 222, andthe activated carbon 226 is provided on the top of the absorbent 224,namely, in the vicinity of an inlet of the container 22. By implementingthis structure, the liquid fuel leaked out of the gas circulation tube200 stays once in the bottom of the container 22, thus inhibiting theleakage of the fuel. Also, the fuel left uncollected by the absorbent isacceleratedly evaporated by the activated carbon having a large surfacearea, and it is evaporated to a degree that the concentration thereofdoes not exceed a predetermined concentration (for example, 200 ppm).

FIG. 7 illustrates another configuration of the vapor-liquid separationstructure 210. In this configuration, a fuel collector 220 is placednear heat generating units such as a fuel cell and a heat exchanger. Inthe fuel collector of this configuration, an activated carbon 226 isprovided around a gas circulation tube 200 inserted into a container222, and absorbent 224 is further provided around the activated carbon.By employing this structure, the liquid fuel having reached the tip ofthe gas circulation tube 200 is evaporated, in an accelerated manner,thanks to the heat of the heat generating units. And it is evaporated sothat the concentration thereof does not exceed a predeterminedconcentration (for example, 200 ppm). Also, the fuel left unevaporatedis absorbed by the absorbent 224, so that the leakage of fuel is hardlylikely to occur.

INDUSTRIAL APPLICABILITY

In the present embodiments, a description has been given of a fuel cellsystem formed by DMFC using the methanol as liquid fuel. However, thetype of a fuel system, for directly supplying the liquid fuel, to whichthe present invention can be applied is not limited to the DMFC system.A description has been given of a fuel cell system of such a form thatuses a personal computer as a load, but the present invention can beapplied to a fuel cell system that can be used in various types ofequipment particularly portable equipment.

1. A fuel cell system that supplies liquid fuel to a fuel cell andoperates the fuel cell, the system including: a first fuel storage,provided detachably, which stores the liquid fuel supplied to the fuelcell; a first fuel supply means which delivers the fuel cell stored insaid first fuel storage; a second fuel storage which stores the liquidfuel delivered by said first fuel supply means; a second fuel supplymeans which delivers the liquid fuel stored in the second fuel storage;and a fuel detection means, provided in said second fuel storage, whichdetects an amount of the liquid fuel stored in said second fuel storage,wherein a free path through which gas inside said second fuel storage issucked in and discharged is provided on top of said second fuel storage.2. A fuel cell system according to claim 1, wherein the free path istube-shaped such that one end thereof is open.
 3. A fuel cell systemaccording to claim 2, wherein the free path is such that inner volumethereof is greater than or equal to one-time liquid cell capacity bysaid first fuel supply means.
 4. A fuel cell system according to claim2, wherein a vapor-liquid separating structure is provided in an end ofthe free path.
 5. A fuel cell system according to claim 3, wherein avapor-liquid separation structure is provided in an end of the freepath.
 6. A fuel cell system according to claim 4, wherein thevapor-liquid separating structure has either an absorber which collectsthe liquid fuel or a filter which absorbs the fuel in exhaust gas.
 7. Afuel cell system according to claim 4, wherein the vapor-liquidseparating structure has either an absorber which collects the liquidfuel or a filter which absorbs the fuel in exhaust gas.
 8. A fuel cellsystem according to claim 6, wherein the filter is activated carbon. 9.A fuel cell system according to claim 7, wherein the filter is activatedcarbon.
 10. A fuel cell system according to claim 1, further including athird fuel storage, connected with said second fuel supply means, whichmixes emission material from the fuel cell with the liquid fueldelivered by said second fuel supply means, which prepares and stored afuel to be supplied to the fuel cell, and which discharges unwanted gascomponent to the outside of said fuel cell system, wherein the end ofthe free path is connected with a gaseous layer part of said third fuelstorage.